1. Field of the Invention
2. Description of Related Art
Electrodialysis uses direct current as a means to cause the movement of ions in solutions. Electrodialysis processes are well known in the art and are typically carried out in a stack arrangement comprising a plurality of flat sheet membranes. The stack consists of electrodes (anode and cathode) at either end and a series of membranes and gaskets which are open in the middle to form a multiplicity of compartments separated by the membranes. Usually, a separate solution is supplied to the compartments containing the electrodes. Special membranes may be placed next to the electrode containing compartments in order to prevent mixing of the process streams with the electrode streams. The majority of the stack between the electrode compartments comprises a repeating series of units of different membranes with solution compartments between neighboring membranes. The repeating unit is called the unit cell, or simply, a cell. Solution is typically supplied to the compartments by internal manifolds formed as part of the gaskets or by a combination of internal and external manifolds. The stacks can include more than one type of unit cell. Streams may be fed from one stack to another in order to optimize process efficiency. Usually the change in composition of a stream after one pass through the stack is relatively small and the solutions can be recycled by being pumped to and from recycle tanks. Addition of fresh solution to and withdrawal of product from the recycle loop can be made either continuously or periodically in order to control the concentration of products in a desired range.
Treatment of aqueous salt streams by electrodialysis to form acid and/or base from the salt is known. The aqueous salt stream is fed to an electrodialytic water splitting apparatus which comprises an electrodialysis stack and a means for electrodialytically splitting water. A useful apparatus is disclosed in U.S. Pat. No. 4,740,281. A useful means to split water to (H.sup.+) and (OH.sup.-) is a bipolar membrane such as disclosed in U.S. Pat. No. 4,766,161. The bipolar membrane is comprised of anion selective and cation selective layers of ion exchange material. In order for the membrane to function as a water splitter, the layers must be arranged so that the anion layer of each membrane is closer to the anode than the cation layer. A direct current passed through the membrane in this configuration will cause water splitting with hydroxyl ions being produced on the anode side and a corresponding number of hydrogen ions being produced on the cathode side of the membrane.
Electrodialytic water-splitting in a two-compartment cell has been disclosed, for example, in U.S. Pat. No. 4,391,680 relating to the generation of strongly acidified sodium chloride and aqueous sodium hydroxide from aqueous sodium chloride. U.S. Pat. No. 4,608,141 discloses a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for basification of aqueous soluble salts. U.S. Pat. No. 4,536,269 discloses a multi-chamber two-compartment electrodialytic water splitter and a method for using the same for acidification of aqueous soluble salts. These two patents review the use of two-compartment electrodialytic water splitters to treat salts.
Three-compartment electrodialytic water splitters are disclosed to be comprised of alternating bipolar, anion and cation exchange membranes thereby forming alternating acid, salt and base compartments. U.S. Pat. No. 4,740,281 discloses the recovery of acids from materials comprising acid and salt using an electrodialysis apparatus to concentrate the acid followed by the use of an electrodialytic three-compartment water splitter to separate the acid from the salt.
The design of membranes for electrodialysis processes is complicated by the conflicting needs of obtaining low electrical resistance, high permselectivity and good mechanical properties. A high concentration of charged groups is necessary to achieve good permselectivity (permeation selectivity to either anions or cations) and considerable water swelling is needed to obtain a low resistance. In order to maintain good selectivity, swelling must be controlled. This can be accomplished by crosslinking the membrane resin or incorporating an inert matrix. Nearly all commercially available ion exchange membranes use a separate reinforcing fabric to support mechanically weak ion exchange material. Reinforced membranes are manufactured by Asahi Glass Co. (Tokyo, Japan) and Tokuyama Soda Co. Ltd. (Tokuyama City, Japan) under the trade name Selemion.RTM. and Neoseptas.RTM.. These are apparently prepared by the paste method as described by Y. Mizutani, R. Yamane, and H. Motomura, Bull. Chem. Soc. Jpn., 38, 689 (1965). A fabric reinforcing material is an integral part of that process. Other widely available membranes, such as those of Ionics Inc. (Watertown, Mass.) and DuPont (Wilmington, Del.), also employ a fabric reinforcement to achieve good mechanical properties. Nafion.RTM. membranes made by DuPont are available with and without fabric reinforcement. The use of a fabric reinforcing material can improve the burst strength of a membrane and especially improve its tear resistance. However, the use of the reinforcing material can cause problems: 1) roughness makes sealing difficult; 2) part of the area is blocked off by the fibers and is not available for transport; 3) cracking can occur along fibers; 4) precipitates can form inside the membrane because of the heterogeneity caused by the fibers.
Therefore it is desirable to use membranes which do not rely on a fabric reinforcement to achieve good mechanical properties. Such membranes can be produced by radiation grafting onto a polymer film with the base film serving to improve the mechanical properties. Such membranes are produced commercially under the trade name Raipore.RTM. by RAI Research Corporation, Hauppauge, N.Y. Other membranes can be prepared by solvent casting a polyelectrolyte and matrix polymer as described in U.S. Pat. No. 4,012,324 or by casting solutions of ion containing polymers with sufficient charge density to be water swollen but with enough water insoluble material to resist swelling. This type of the cation membrane is described in U.S. Pat. No. 4,738,784. Similar anion and bipolar membranes are described U.S. Pat. No. 4,116,889 and U.S. Pat. No. 4,766,161.
The design and operation of electrodialysis stacks is well known in the art and is described, for example, by H. Strathmann in the chapter "Electrodialysis" in Synthetic Membranes: Science, Engineering and Applications, P. M. Bungay, H. K. Lonsdale, M. N. dePinho Editors, D. Reidel Publishing Co., Dordrecht, Holland. Careful design of the electrodialysis stack hardware and operation to minimize pressure differentials and surges can help reduce the failure rate in membranes which are not mechanically robust. Membranes which are not fabric reinforced generally will not fail in use, but should a membrane fail, it usually results in a large tear or hole because of the poor tear resistance of membranes which do not include a reinforcement. The resulting hole causes a leakage of solution between adjacent compartments and usually causes a loss of current efficiency, product concentration, and/or purity. The problem is especially acute when dealing with the concentrated solutions usually found in electrodialytic water splitting processes such as those described in U.S. Pat. No. 4,740,281, U.S. Pat. No. 4,504,373 and U.S. Pat. No. 4,557,815. The mixing of acid and base caused by failure of a bipolar membrane in an electrodialytic water splitting stack can greatly reduce process efficiency. If even one bipolar membrane in a stack of 100 rips, it is usually necessary to disassemble the stack and replace the broken membrane. Even though the cost of the membrane may not be significant, it is highly desirable to avoid shutting down the plant to perform this time consuming procedure. Reinforced bipolar membranes would be helpful, but the production of such membranes which retain the other desirable properties has proven difficult. In pressure driven membrane processes, it is well known to use a porous support to enhance the mechanical performance of otherwise weak membranes. Such structures have generally not been used in electrodialysis, probably because of problems with concentration polarization and delamination caused by differences in water transport between the layers.
It is desirable to minimize the effect of broken membranes on process efficiency and increase the interval between plant shutdowns caused by such leakage. It is also desirable to provide a system capable of improving the electrical performance of bipolar membranes.
U.S. patent application Ser. No. 590,116 discloses using a guard membrane oriented adjacent to and in contact with a first membrane. The guard membrane preferably has the same ion selectivity as the guard membrane and acts as a backup membrane should the first membrane break during use.
Further, it has been found that in the electrodialysis of weak acid systems guard membranes which are adjacent to the first membrane "separate" from the bipolar membranes. This results from formation of a non-conducting layer of weak acid or a bubble between the guard and the first membrane. The non-conducting layer greatly increases the potential drop across the cell to excessively high levels. For guard membranes to be practical in weak acid systems it is necessary to find guard membranes which will not separate from the first bipolar membrane during use.